I can pretty much be certain that the most appropriate method of examining the cards and using as a reasonable definition is the following...
deprecated/Outdated - These are the cards that will never see updates... This will pretty much Cover the "5xx and earlier, rs6xx/rs740 and earlier".
Legacy - These cards are older, and are not as likely to see an update, however there is a chance to see fixes on these devices. This covers the "6xx/7xx/rs7xx/rs8xx"
Current - Currently supported cards that are supported with monthly updates... This includes Evergreen and newer cards.
Given the list of cards here, there is one major fact that can be shown. It's impossible to mix and match these cards reasonably given the current driver status. The fglrx driver currently prevents developers from having a mixed configuration. I personally would love to be able to have a single system setup with a Radeon X1900GT (R580), a Radeon HD4550 ( RV710 ), and a Radeon HD 5770. I know that it is easy enough to find mother boards where this type of configuration can happen, however, the current drivers are lacking in the sense that all three are unable to be used at the same time.
Using this, I figure that most end users would actually expect linux to respond similarly to what would happen on windows. This would mean that all three cards can be used at the exact same time without failure ( Given what happens with windows 7 and Windows XP ).
Also, Please keep in mind That this is where on the given system all three displays have their own monitor attached. If anything, There should be improvements to allow the Open Source drivers and closed source drivers to coexist within the system. This would mean that improving the driver to use XRandR 1.5, and Possibly moving to instead make use of the kernel drm drivers ( This would mean deprecating all kernels released before kernel 3.1, since kernel 3.1 was when the symbols in drivers/gpu/drm where changed from EXPORT_SYMBOL_GPL to EXPORT_SYMBOL ). Also other kernel changes might make it more important to improve interoperability also...The 3.4 kernel release added DRM_prime to the DRM system. This in theory will allow much better support Crossfire, and CrossfireX.